We have previously demonstrated that activated cellular Harvey (H) ras oncogene can confer the metastatic phenotype upon non-neoplastic NIH/3T3 cells through DNA transfection. The possibility was examined whether the metastatic behavior was associated with amplification or increased expression of the ras gene. An autochtonous NMU-induced rat mammary carcinoma that possesses activated Harvey ras was used for this purpose. Nonmetastatic vs metastatic primary tumors were examined, as well as primary tumors vs metastases. There were no differences between the ras specific DNA and RNA levels of the nonmetastatic and metastatic tumors. Ten individual lung metastases and a single primary tumor were resected from a Sprague-Dawley rat, and were expanded through subcutaneous transplantation in nude mice. There was a variability in the ras expression among the lung metastases. Some were several fold higher than the primary and three of the ten metastases had barely detectable RNA levels. Southern blot of EcoR1 restricted DNA from the primary rat tumor and pooled fragments of multiple lung metastases showed about ten-fold higher ras DNA levels in the primary tumor. However, after multiple passages of NMU tumors in syngeneic rats, no differences in the ras DNA levels were detectable. These data imply that H-ras gene amplification or increased expression is not necessary for established metastases. The primary S.D. rat NMU tumor exhibited additional H-ras sequences in PVU-II cleaved DNA that were not seen in normal rat mammary glands, or the nude mouse transplants of the same tumor.